For light emitting devices, such as light emitting diodes (LEDs) and especially deep ultraviolet LEDs (DUV LEDs), minimizing a dislocation density and a number of cracks in the semiconductor layers increases the efficiency of the device. To this extent, several approaches have sought to grow low-defect semiconductor layers on patterned substrates. These approaches typically rely on reducing stresses present in epitaxially grown semiconductor layers.
For example, one approach to reduce stress accumulation in an epitaxially grown layer relies on patterning the underlying substrate using microchannel epitaxy (MCE). Using MCE, a narrow channel is used as a nucleation center containing low defect information from the substrate. An opening in a mask acts as a microchannel, which transfers crystal information to the overgrown layer, while the mask prevents dislocations from transferring to the overgrown layer.
Other approaches rely on epitaxially growing a group III nitride based semiconductor superlattice. The superlattice structure mitigates the strain difference between an aluminum nitride (AlN)/sapphire template and the subsequent thick AlxGa1-xN (where 0≦x≦1) layers. For devices such as DUV LEDs, thick AlGaN epitaxial layers (e.g., of the order of a few micrometers) are desirable to reduce current crowding. Using a superlattice approach, an AlN/AlGaN superlattice was grown to reduce biaxial tensile strain and a 3.0-μm-thick Al0.2Ga0.8N was grown on sapphire without any cracks. Similarly, a superlattice structure shown in FIG. 1A can comprise a periodic structure with each element 2A-2D composed of alternating sublayers of semiconductor materials with different polarizations and different accumulated stresses in the sublayers. Such a superlattice can be used to minimize the dislocation density due to varying stresses in the sublayers of the superlattice elements.
While the superlattice approaches allow some control of tensile and compressive stresses in epitaxially grown nitride semiconductor layers, the approaches do not enable epitaxial growth of nitride based semiconductor layers with uniform composition. To grow such layers, variation of nitrogen and aluminum vacancies has been explored. For example, a migration enhanced metalorganic chemical vapor deposition epitaxial growth technique (with an NH3 pulse-flow) can be used to grow high-quality AlN layers. Variation of growth modes can be used to reduce threading dislocations. Additionally, FIGS. 1B and 1C illustrate another approach for fabricating AlN multilayer buffers according to the prior art. In this case, a pulsing NH3 gas flow rate is used to control crack propagation and threading dislocations in the semiconductor layers. FIG. 1B shows the gas flow sequence used for NH3 pulse-flow growth, while FIG. 1C shows a schematic structure of the AlN buffer. In a first step, an AlN nucleation layer and an initial AlN layer are deposited using NH3 pulse-flow growth. A low threading dislocation density was achieved by a coalescence process of the AlN nucleation layer. For example, as observed from a cross-sectional transmission electron microscope (TEM) image, edge-type and screw-type dislocation densities of an AlGaN layer on an AlN buffer layer were reported as 3.2×109 and 3.5×108 cm−2, respectively.
In another approach disclosed in U.S. Pat. No. 8,080,833, relaxation of some semiconductor layers has been proposed. In particular, this approach seeks to relax the p-type contact layer due to a high GaN content, and therefore a high lattice mismatch. FIG. 1D shows the proposed schematics of the layer, where defects 3A-3C are shown within the relaxed p-type cap layer 4.